For small generators in the power generation industry, particularly those that use air cooling, excitation requirements are relatively small, e.g., generally under 1 Megawatt of excitation power, and brushless excitation provides a cheaper and simpler means of providing excitation for a steam turbine generator. A brushless exciter conventionally includes a direct current ("DC") field winding, a main revolving armature alternating current ("AC") exciter, and a diode or rectifier wheel. The field of a rotating permanent magnet generator ("PMG"), rather than an electromagnet, is often used for the primary excitation. The pilot exciter AC output is rectified and DC power is provided to the brushless exciter field winding. The pilot exciter thereby eliminates the need for a continual external energy source. The brushless system eliminates the need for brushes and current collection components.
An example of such a shaft-driven, brushless excitation system 10 is shown in FIG. 1 where a combustion turbine 12 is connected to a combustion turbine generator ("CTG") 15 along a common shaft 11. A starting package 14 is conventionally used in such a system 10 and is also connected by the common shaft 11 to the brushless excitation system 10. A clutch 13 or torque converter is normally connected to the shaft 11 as illustrated as well and as understood by those skilled in the art. A CTG 15, however, does not have the capability to start itself so it requires some external means of starting or a starting package such as either a motor or a static starting system.
Static exciter starting systems, such as shown in FIG. 2, require that the generator 15' have field excitation. The static excitation systems can also be quite expensive. In other words, the prior art system as shown in FIG. 2 includes a static start 14' and a static excitation system 10' which has brushes or brush gears connected to the generator 15' along the common shaft 11' which also connects to the turbine 12'. A common reason given for not employing brushless excitation 10 in CTGs is that brushless excitation systems 10 are not compatible with static start 14'. This incompatibility is primarily due to the fact that the stationary field used by the existing design of brushless exciters in turbine-generators is a direct current ("DC") field. A DC field induces no voltage in a stationary armature so the generator 15 receives no field current at zero speed and cannot be started as a synchronous motor as required in a static starting system.
Static starting also requires that excitation be provided to the generator 15' at all speeds from zero to synchronous speed. At zero speed, a DC field voltage generates no voltage in a rotation armature so there is no field current supplied to the generator. Accordingly, the use of DC brushless excitation with static start has previously been confined to steam turbine-generators and motor start CTGs.